The present invention relates to the general field of systems for injecting an air/fuel mixture into a turbomachine combustion chamber. More particularly, it relates to an injection system of the aerodynamic type provided with means for creating effervescence in the fuel prior to it being mixed with air.
The conventional process for designing and optimizing a turbomachine combustion chamber seeks mainly to reconcile implementing operational performance of the chamber (combustion efficiency, stability domain, ignition and re-ignition domain, lifetime of the combustion area, etc.) as a function of the intended mission for the airplane on which the turbomachine is mounted while minimizing emissions of pollution (nitrogen oxides, carbon monoxide, unburnt hydrocarbons, etc.). To do this, it is possible in particular to act on the nature and the performance of the injection system for injecting the air/fuel mixture into the combustion chamber, on the distribution of dilution air inside the combustion chamber, and on the dynamics of air/fuel mixing within the combustion chamber.
The combustion chamber of a turbomachine typically comprises an injection system for injecting an air/fuel mixture into a flame tube, a cooling system, and a dilution system. Combustion takes place mainly within a first portion of the flame tube (referred to as the “primary zone”) in which combustion is stabilized by means of air/fuel mixture recirculation zones induced by the flow of air coming from the injection system. In the second portion of the mixer tube (referred to as the “dilution zone”), the chemical activity that takes place is less intense and the flow is diluted by means of dilution holes.
In the primary zone of the flame tube, various physical phenomena are involved: injection and atomization into fine droplets of the fuel, evaporation of the droplets, mixing of the fuel vapor with air, and chemical reactions of the fuel being oxidized by means of the oxygen in the air.
These physical phenomena are governed by characteristic times. Atomization time thus represents the time needed by the air to disintegrate the sheet of fuel to form an air/fuel spray. It depends mainly on the performance and the technology of the injection system used and on the aerodynamics in the vicinity of the sheet of fuel. Evaporation time also depends on the injection system used. It is a function directly of the size of the droplets resulting from the disintegration of the sheet of fuel; the smaller the droplets, the shorter the evaporation time. Mixing time corresponds to the time needed for the fuel vapor coming from the evaporation of the droplets to mix with the air. It depends mainly on the level of turbulence inside the combustion area, and thus on the flow dynamics in the primary zone. Chemical time represents the time needed for the chemical reactions to develop. It depends on the pressures and temperatures at the inlet to the combustion area and on the nature of the fuel used.
The injection system used thus plays a fundamental role in the process of designing a combustion chamber, in particular when optimizing the times that are characteristic of fuel atomization and evaporation.
There exist two main families of injection systems: “aero-mechanical” systems in which the fuel is atomized as a result of a large pressure difference between the fuel and the air; and “aerodynamic” systems in which the fuel is atomized by being sheared between two sheets of air. The present invention relates more particularly to such aerodynamic systems.
Aerodynamic injection systems known in the prior art present numerous drawbacks. In particular, at low turbomachine speeds, fuel atomization becomes highly degraded, thereby decreasing the stability of combustion and running the risk of the combustion area going out while also increasing polluting emissions of the nitrogen oxide type.